It is necessary in many compressor applications to maintain both the intake pressure (inlet pressure) and the end pressure (outlet pressure) of the compressor at a constant value. The control elements used for this are a throttling member on the intake side of the compressor as well as control units, with which the position of the guide vanes and/or the speed of rotation of the compressor can be changed. The compressor delivers the compressed gas with an end pressure that is to be held at an essentially constant pressure to a user or to a process in which the gas is processed. This user of this process usually has control units of its own for changing or controlling the gas throughput, e.g., a throttling member at the inlet of the user or process.
It has been known that the compressor may be equipped with at least three control circuits to maintain the intake pressure and the end pressure at a constant value and to prevent the flow instabilities. An end pressure controller usually acts on the guide vanes of the compressor and/or on a means for adjusting the speed of rotation. An intake pressure controller acts on the throttling member on the intake side of the compressor. Furthermore, a pump surge limiter is provided, which acts on a blow-by valve connecting the outlet side to the intake side of the compressor in order to ensure a minimum flow rate through the compressor in the case of excessively low flow rate of the compressor by blowing over flow media from the delivery side to the intake side.
Each of these three control circuits affects the other. The absence of instabilities is ensured only by a careful and mutually coordinated design of the control circuits.
The working points and the dynamic behavior of the control circuits are designed for a normal value of the flow rate of the compressor and consequently of the gas throughput at the outlet of the compressor. Even though the control circuits are able, if a sudden change occurs in the gas throughput needed on the user side, to follow such changes in the gas flow rate to a certain extent, they are inherently able to do so with a sluggish time response only, which it not sufficient for a rapid elimination of changes or disturbances in the user-side gas output and may also cause the control circuits to mutually affect one another and to engage in an undesired interaction.